Jimmy Fallon (Lloyd Bishop/AP)

For the second time in about four years, Jay Leno is stepping down from NBC’s iconic late night franchise “The Tonight Show” — this time in spring 2014 to make way for “Late Night” star Jimmy Fallon.

Fallon will take over sometime after that, the network announced Wednesday, confirming the industry’s worst kept secret in recent history.

NBCUniversal CEO Steve Burke explained Wednesday  the timing is intended to mesh with NBC’s broadcast of the Winter Olympic Games from Socchi, Russia, which we swear is the truth and not a gag from an “SNL” skit, like it sounds.

“Saturday Night Live” impresario Lorne Michaels is taking over as executive producer; Debbie Vickers, who has produced the show for the past two decades, will exit along with Leno. And the show will move back to its original home at 30 Rock in New York City, where it was based until Johnny Carson, the franchise’s longest running host at 30 years, moved it to Burbank permanently in May of 1972.

“We’re thrilled ‘The Tonight Show’ is returning home to New York City, and it’s the perfect symbol of incredible comeback we’ve worked to create in our city’s film and television industry,” Mayor Bloomberg weighed in on Wednesday, followed by a bunch of political “not since the invention of television has so much production been based in our city” blah, blah, blah,  and finally running out of gas with, “and we couldn’t be happier that one of New York’s own is bringing the show back to where it started — and where it belongs.”

There’s no news yet on a replacement for Fallon at 12:35 a.m. NBC said coyly in its announcement that “programming plans” for the time period are in development and will be announced soon. Under consideration is the return of “Tonight Show” to its early, 90-minute format. (“Tonight Show,” which debuted in 1954, with Steve Allen as host, was cut down to one hour during Carson’s long run.)

“We are purposefully making this change when Jay is #1, just as Jay replaced Johnny Carson when he was #1,” Burke said in Wednesday’s news.

There’s one big difference: Carson shocked NBC suits in May of 1991 when he announced his retirement at an affiliate conference in New York. (That, following  press reports NBC was concerned that Carson (who was in his mid-60’s) was losing younger viewers, and that NBC had guaranteed Leno the gig when Carson retired.

Jay Leno (Paul Drinkwater/AP)

Leno, 62, on the other hand, is being told when to step down — again. Leno’s contract expires in ’14.

Last time Leno was informed he was stepping down as host of “Tonight,” it was to make way for then “Late Night” host Conan O’Brien. NBC made that announcement in fall of 2004, having negotiated a contract to hang on to Conan by promising him he’d replace Leno on “Tonight” in June of 2009 and, well, you know the rest.

In Wednesday’s announcement about Fallon, NBC made no mention of Conan by name as having interrupted Leno’s run for seven months back then, when NBC parked Leno at 10 p.m. weeknights, where he provided Conan with lousy lead-in ratings (including some numbers NBC would be happy to have at 10 p.m. these days).

Leno, you’ll recall, regained the keys to “Tonight Show” after NBC decided to push the Conan-hosted show’s start time to 12:05 a.m. in order to squeeze Leno back into late night in some newly-named half hour late night program.

Hey — here’s a bit of irony! The NBC suit who’d announced that plan  to TV critics attending Winter TV Press Tour 2010, said they hoped all the deals for the change would be done in time to mesh with NBC’s broadcast of the Winter Olympics!

 Conan, however, did not sign, saying he’d rather walk than see “Tonight Show” moved to 12:05 a.m. And he did.

NBC’s only reference to the Conan years is that the show hosted by Leno, who took over the show when Carson retired in ’92, has ranked No. 1 ahead of its ABC and CBS time-period competition in 18-49 viewers and total viewers for each season since Leno’s return in March 2010 and for the 14 seasons before his departure in May 2009.

In fact, NBC says “Leno has hosted ‘The Tonight Show with Jay Leno’ since taking over the reins from television icon Johnny Carson in 1992.”  All true — since Conan hosted “The Tonight Show with Conan O’Brien.”

 Leno’s not being moved out because he’s failing in the ratings. During the current season, “Tonight” is consistently beating ABC and CBS timeslot rivals among the 18-to-49-year-old viewers who are the currency of NBC’s ad sales; Leno’s also first among viewers of all ages, regularly attracting an average of 3.5 million.

 But Leno’s in his 60’s, and when ABC moved 45-year-old Jimmy Kimmel into Leno’s 11:35 p.m. timeslot, last January, the press began speculating that Kimmel would scoop up all the young viewers in the timeslot before Leno’s heir-apparent, Fallon, 38, would eventually move in to the time period. (Since his debut in the earlier timeslot in January,  Kimmel has achieved a slight lead over Leno among 18-to-34-year-old viewers — averaging 332,000 and 319,000 of them, respectively.)

Kimmel, who, with Leno’s exit, will lose his favorite punching bag, tweeted, “congratulations to my dear, sweet @jimmyfallon — a formidable rival and an incredible lover.”

Ironically, Fallon, with his broader, sweeter humor, may be better suited to take over for Leno than was Conan because of his greater appeal to older viewers. When Conan took over “Tonight” he’d lost a lot of his edge over CBS competitor Craig Ferguson among total viewers.

 At the end of the 06-07 season, Conan had a 24 percent lead over Ferguson (2.308 million vs. 1.856 million). When Conan ended his “Late Night” run, in February of ’09, his margin over Ferguson was down to 4 percent (2.000 million vs. 1.923 million).

Since Fallon’s arrival, “Late Night” is once again more consistently beating Ferguson in total viewers.

Leno did not go gently into the night. He’d instead been savaging NBC in his opening monologues pretty regularly in the days leading up to NBC’s announcement.

But earlier this week, signaling an announcement was imminent, he and Fallon teamed up for a duet, which became  a YouTube sensation, of course.

“Hey kid, how you holding up?” Leno was seen asking Fallon over the phone, just after the camera has shown Leno sadly exiting the stage after a show, and Fallon forlornly looking out his office window, surrounded by newspaper clippings of “Tonight Show” war articles.

“Ok, I guess. You?” Fallon replied.

“Yeah, I’ll live. I’ve been through this before,” Leno sighed. “Gotta admit, I’m getting a little sick of all of this.

Fallon looked troubled. “Jay, can I ask you something? We’re still friends, right?”

“Yeah, of course we’re still friends,” Leno assured him.

“That's good,” Fallon said, relieved.

Cue duet of “West Side Story” tune “Tonight” with lyrics re-written as befitted the occasion.

TV critics hated it – real schmaltz, they scoffed, nicking hip Fallon for having caved in to pressure to do the video. Hollywood Reporter’s Tim Goodman, for instance,  snarked,  “the video isn't funny. It's too long. Doesn't have an ounce of believability and plays like a clip from a Hallmark movie about lovers that's airing sometime soon on Bravo. And it makes Fallon look like simpering suck-up who sold out because he was probably told to.”

Then came word it had been Fallon’s idea. Oops.

“Congratulations Jimmy. I hope you’re as lucky as me and hold on to the job until you’re the old guy,” Leno said in Wednesday’s announcement. “If you need me, I’ll be at the garage.”

Added Fallon: “I’m really excited to host a show that starts today instead of tomorrow.”